As is known in the art, it is sometimes desirable to form Group III-V devices on a substrate as a Monolithic Microwave Integrated Circuit (MMIC). One such substrate is silicon, as described in the above-referenced copending U.S. patent application Ser. No. 14/105,497, filed Dec. 13, 2015, and other substrate is a silicon carbide (SiC) wafer, for example a four inch wafer of silicon carbide, having a thickness of about 400-500 microns, with a semiconductor layer of Group III-V material, such as GaN epitaxial layers formed on the upper surface using MOCVD or MBE.
As is also known is the art, microstrip transmission lines are sometimes used to interconnect active devices, such FET devices, and passive devices formed on, or in, the Group III-V layer. In one such case, after forming the FET and strip conductors of the microstrip transmission lines, the 400-500 micron thick SiC wafer must be thinned or polished to a thickness of 50-100 microns for: the ground plane conductor microstrip transmission line which will be formed on the backside of the silicon carbide wafer; and, to accommodate conductive vias which pass through the silicon carbide wafer from the ground plane to electrodes of the FET. The process of thinning or polishing the backside of the wafer and formation of the vias from the backside of the wafer, however, are difficult to control. Further, the high cost of bulk SiC wafers acts as an impediment to wafer scaling (in the near term) as a 200 mm wafer will necessarily have to be thicker than a 150 mm wafer or 100 mm wafer in order to mitigate potential wafer breakage.
As is further known in the art, ohmic contacts are required in forming source and drain contacts for the FETs. To form these ohmic contacts, a rapid thermal anneal (RTA) process is used to anneal the contact metal to the semiconductor layer. The RTA process typical uses optical lamps to heat the surface of the wafer with the contact metal on the portions of the semiconductor layer where the ohmic contacts are formed. Because the SiC and GaN are optically clear (wide bandgap), absorption of the energy during rapid thermal anneal (RTA) of metal source and drain contacts becomes front mask/pattern dependent. This in turn leads to inconsistent ohmic contact results. Various approaches have been tried to deal with this issue. In one commonly used technique, the wafers are placed in a graphite susceptor which is heated by the lamps and the heat is absorbed by the susceptor is conductively transferred to the wafer to thereby heat the metal contact and form the requisite ohmic anneal. However, front mask/pattern dependent and generally inconsistent Ohmic contact results still often occur. Additionally, in volume production environments with large diameter wafers (e.g. 200 mm) susceptor use is impractical.